The present invention is directed to a system for transmitting data between a plurality of data processing units and more particularly to a communications controller which includes a contention circuit to award priority of access to a communications channel among a plurality of requesting processing units.
In data processing systems in which a central processing unit is connected over a common communication channel to a plurality of peripheral devices, access to the channel is given to the first processing unit requesting access. When two or more processing units attempt to gain access at the same time, access is awarded based on a priority arrangement. Prior art priority arrangements have been based on the address of the requesting processing units in which the address of the requesting units are required to be transmitted to a communication controller which compares the addresses to determine which processor unit has priority of access to the channel. The data is then required to be transmitted by the winning processing unit. This type of processing operation increases the time to transmit the data between the processing units. It is therefore the principal object of this invention to provide a contention circuit which will automatically award priority of access to a requesting processing unit without requiring the transfer of any data between the requesting processing units. It is a further object of this invention to provide a LSI semiconductor chip for controlling the transmission of data between a plurality of processing units.